Hi there,
this is mold and mildew growing in the ducts, have the drain tube checked for restrictions as this may be the cause of excess moisture buildup with in the system, and there is a duct treatment called clean air, this will eliminate the problem growth, you should be able to get this from any spare parts outlet.
Mark (mhpautos)
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Wednesday, October 7th, 2009 AT 4:51 PM